It's a rainy night in Gotham, and some of the GCPD's finest are being lead by three particularly crooked cops, through the sewers, in search of something. But little do they know, while they think they may be on the hunt, Killer Croc happens to be hunting them in the shadows.
While Croc ambushes the platoon of cops, taking every nameless cop as well as one of the three leading them (of course, the black guy dies first), we get a little of his past. First, before he turned into the monster he is now, he was a poor kid with a skin condition, which his unsympathetic aunt tried to "help" him with, by trying to scrub his skin off with what looked to be a pumice stone, or something pretty rough like that. Flash forward a handful of years, and Croc is being stiffed on the freak show circuit, by a ring leader, which leads to him quite literally biting the hand that feeds. Then a couple of years ago, he returns to Gotham, and doesn't do quite well at the crime thing, leaving him to be arrested and berated by Jason Todd as Robin.
Back in the sewer, the two cops who managed to escape Croc's attack find a group of people down one of the tunnels, and ask for help, but turns out, these people are working for Croc, as a little girl breaks the lady cop's leg, then the others cut the lights in the tunnels. The two cops come to a break in the path, which means the one with no broken leg has to jump to scout ahead, and finds one of his former coworkers' dead body waiting for him.
Turns out these three were quite dirty, dealing in Gotham turf by the docks. One of their fellow cops was thinking about turning them in, but that just meant he got a bullet to the head and was dumped in the river, where Croc's followers retrieved the body. The whole reason why these cops are in the sewer is because they got a note saying that someone has evidence against them, and to bring money. They figure it may have been in the cop they killed's safe box, and are hoping to get the key. Back in the present, the cop investigating the body finds the key, and turns around with the good news, only to see through his night-vision goggles, Croc waiting behind his partner... then comes the chomp.
So the last man left goes running, as Croc stalks him through the tunnels, and of course, because the dude doesn't know where he's going, he falls down a bit, and gets impaled on some stuff. In true horror fashion, as Croc approaches slowly for the kill, the man keeps asking what this is about, but Croc doesn't answer.
In his monologue, Croc remembers how everyone in his life turned their back on him, and it turns out, this cop knew Croc as a kid, and was pretty nice to him, one of the only people to actually be nice to him.
So with his abandonment issues, Croc returns to his followers, and promises never to abandon them, for they own the biggest territory in Gotham, the underground, and he'll lead them to glory. All of Croc's followers definitely take part in the kool-aid, and start chanting "King Croc!" so loudly, that it can be heard from the streets above.